The long-term goal of this laboratory is to understand how biomechanics, anatomy, and physiology enable hearing of high sensitivity and directional acuity. This proposal focuses on microscale ears because physical constraints make sound localization and high sensitivity extraordinarily challenging with ears of microscale dimension. Mosquitoes are among the smallest animals that hear. They possess antennae attached to Johnston's hearing organs (JOs) as external appendages on their heads. JOs are functionally analogous to the mammalian cochlea, but far more accessible. This work will use Doppler laser vibrometry to measure mechanical responses and physiological recordings to test the sensitivity of the two antennae. Preliminary data show that these ears are sensitive to sound over a large bandwidth and have modes of vibration substantially different from those published before. These data will lead to a mathematical model of antennal vibration. Preliminary calculations already indicate that antennae may have noise characteristics superior to those of tympana. The Aedes mosquito is not the smallest insect that hears. Interestingly, the antennae of even tinier midges and of even larger mosquito species are all about the same size. This suggests that they are all near the lower size limit for acoustic function. Comparing ears in these three species may reveal the limits of auditory function. The over 30,000 sensory cells of the JO have a unique and highly-ordered architecture that suggest tonotopic organization and modality fractionation enabling performance equal to macroscopic ears. Mosquito antennae respond to gravity and wind as well as to sound. The basic structure and function of antennae will be probed with molecular markers to determine functional subdivisions among its mechanosensory neurons. This research is relevant to public health and the mission of NIDCD. Far-field particle velocity and antennal ears are a relatively unexplored realm of auditory acoustics at small size. A goal of this work is to reveal the basic principles behind auditory function at all size scales. Miniaturization is coveted goal in practical engineering. The design features of these miniature ears could motivate mechanical engineers to biomimic and translate them into novel nano-to-microscale directional microphones. These findings potentially extend well beyond mere entomological curiosity.